1) Field of the Invention
The present invention relates to a sheet conveying unit that conveys a sheet of recording medium in an image reading unit, a post-processing unit, and an image forming apparatus.
2) Description of the Related Art
In an image forming apparatus such as a copying machine, for example, a sheet material is placed on a sheet conveying guide, the material is conveyed while being guided by the sheet conveying guide, a toner image formed, for example by an image forming unit, is transferred on the sheet material to form an image on the sheet material, and the sheet material with the formed image is discharged to the image forming apparatus main unit to be stacked on, for example, a tray.
Recently, however, for the purpose of resource saving or cost reduction, recycled paper or the back side of used paper is frequently utilized. When an image forming apparatus is used for such recycled sheets of paper over a long period of time, it often results in deposition of paper dust generated in the course of conveyance of sheet materials on a sheet conveying guide.
Particularly, when the sheet conveying guide is provided with a curved portion, since a sheet material hits the sheet conveying guide at the curved portion, or it is bent or rubbed by the curved portion, paper dust adhered to an end surface (a cut edge face) or a surface of the sheet material drops and deposits, particularly on the curved portion or the like.
When paper dust is deposited on the sheet conveying guide as described, the paper dust adheres to another sheet in conveyance again, so that a frictional coefficient of a conveying roller is lowered and a deviation occurs in conveyance of the sheet material.
When deposited paper dust adheres to a sheet material in conveyance again to enter in an image preparing unit, for example, in an electrostatic photography type image forming apparatus, there is a problem of occurrence of such an abnormal image that, when paper dust adheres on an image carrier and the adhered paper dust cannot be completely removed by a cleaning unit, black or white stripes or black or white spots appear on a formed image.
Therefore, a technique for forming a hole for paper dust drop in a sheet conveying guide to allow paper dust to drop from the hole has been proposed (see, for example, Japanese Patent Application Laid-open No. H7-215523).
In the technique, paper dust on a sheet dropping from the hole in the course of conveyance can be removed, however, paper dust which do not drop from the sheet and remains in adhesion thereon cannot be removed and it may drop or deposit in the course of following conveyance of the sheet, which results in contamination inside the image forming apparatus, which further causes lowering of a conveying performance due to skew or increase in slippage rate. In this image forming apparatus, there is such a drawback that the remaining paper dust adheres on an image carrier, or that black or white stripes or black or white spots appear on the image, if the remaining paper dust cannot be completely removed by a cleaning unit.